Method for generation of acoustic vibrations and source of acoustic vibrations for realizing same

ABSTRACT

A method for generation of acoustic vibrations based on shock excitation of a magnetostriction transducer by a pulse electrical signal. The electrical signal is generated in the form of unidirectional half-cycles of cosinusoidal voltage, with a duration from one to two half-cycles of acoustic vibrations produced by the loaded tansducer. The repetition frequency of the electrical pulses is taken to be equal to, or multiple of the frequency of acoustic vibrations. 
     A source of acoustic vibrations comprises a power unit (7), a pulse repetition frequency control unit (10) and a reservoir capacitor (6), the plates whereof are connected through a power circuit of a switching element (5) to a field winding (2) of the magnetostriction transducer (1). The source also comprises an auxiliary field winding (3) disposed on the same magnetostriction transducer (1) and connected by the aiding connection method to the winding (2); an auxiliary switching element (8); and a switching element control unit (9). The auxiliary winding (3) is connected to the plates of the capacitor (6) through a power circuit of the switching element (8) and through the power unit (7), and an output of the pulse repetition frequency control unit (10) is connected to an input of the switching element control unit (9), the outputs whereof are connected to control circuits of the switching elements (5 and 8), respectively. 
     The source of acoustic vibrations is designed primarily for ultrasonic descaling of heat-exchange apparatus.

TECHNICAL FIELD

The present invention relates to ultrasonic engineering, and, moreparticularly, to a method for generation of acoustic vibrations and to asource of acoustic vibrations.

BACKGROUND ART

Known in the art is a method for generation of acoustic vibration pulsesbased on shock excitation of a magnetostriction transducer described inBritish Pat. No. 646,882, published in 1950. This method for generationof acoustic vibrations includes a charged capacitor which is dischargedthrough a winding of the transducer.

There is also known a method for generation of acoustic vibrations,whereby a magnetostriction transducer is excited by pulses of currentsupplied to the winding thereof at a repetition rate 3 to 10 times lowerthan, and multiple of the natural frequency of the magnetostrictiontransducer, with the pulse duration not exceeding 1/2 of the acousticvibration period (cf. USSR Inventor's Certificate No. 251,287, datedJuly 1, 1968). The spectral composition of exciting signals used inembodiments of both methods mentioned above does not provide for fullutilization of the magnetostrictive properties of the materialincorporated in the transducer, hence, the amplitude and power ofacoustic vibrations generated by the prior-art methods are insufficientfor carrying out the major part of production processes.

A pulse source of acoustic vibrations is known (cf. British Pat. No.646,882, published 1950) which serves for preventing scale formation inthermal generating units. The source incorporates a magnetostrictiontransducer, a mechanical switching element, a reservoir capacitor, apower unit, and a pulse repetition frequency control unit.

The prior-art source employs the above-mentioned method for generationof acoustic vibrations, whereby the previously charged reservoircapacitor is discharged through the magnetostriction transducer winding,and inherits all the disadvantages of the method described above.Moreover, the source is characterised by low response, and the powerthereof is limited because of the use of the mechanical switchingelement in the source circuit.

Another source of acoustic vibrations known in the art comprises a powersource, a pulse repetition frequency control unit connected thereto bythe input thereof, and a reservoir capacitor. A field winding of amagnetostriction transducer is connected to plates of the capacitorthrough a power circuit of a switching element (using a thyristor) (cf.USSR Inventor's Certificate No. 575,144, dated Oct. 5, 1977). Theprior-art source inherits a low efficiency resulting from poorexcitation of the magnetostriction transducer. The vibration amplitudeof the magnetostriction transducer of the above source is limited to astatic magnetostriction value and is not higher than 1 to 1.4 μm on avibration frequency of 20 kHz. Therefore, the amplitude of ultrasonicvibrations cannot be raised by increasing the amplitude of theelectrical signal pulse serving to excite the transducer above adefinite level depending on saturation of the given material.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a method of generation ofacoustic vibrations which will permit maximum utilization of themagnetostriction properties of material whereof the transducer core ismade, and, hence, an increase in the amplitude and power of acousticvibrations, and also to provide a source of acoustic vibrations forrealizing same, characterized by simple design and a high efficiency.

This object is accomplished by a method for generation of acousticvibrations based on shock excitation of a magnetostriction transducer byan electrical pulse signal, wherein the excitation pulse signal isgenerated in the form of unidirectional half-cycles of a cosinusoidalvoltage with a duration (d) from one to two acoustic vibrationhalf-cycles of the loaded transducer (assuming P is the period durationof the acoustic vibration of the loaded transducer, and P_(1/2) is thehalf-cycle period thereof, P_(1/2) <d≦P or (1/2)P<d≦P), with arepetition frequency of the cosinusoidal half-cycle voltage (f_(v))equal to, or multiple of the acoustic vibration frequency (f₁), suchthat nf_(v) =f₁.

With this object in view, a source of acoustic vibrations is hereinproposed, comprising a power unit, a pulse repetition frequency controlunit and a reservoir capacitor, the plates whereof are connected througha power circuit of a switching element to a field winding of amagnetostriction transducer, which source is provided, according to theinvention, with an auxiliary field winding disposed on themagnetostriction transducer and connected by the aiding connectionmethod to the main field winding, with an auxiliary switching elementand with a switching element control unit, wherewith the auxiliary fieldwinding is connected to the reservoir capacitor plates in series througha power circuit of the auxiliary switching element and through the powerunit, and an output of the pulse repetition frequency control unit isconnected to an input of the switching element control unit, the outputswhereof are connected to respective control circuits of the main andauxiliary switching elements.

The method for generation of acoustic vibrations realized in accordancewith the present invention provides for maximum utilization of themagnetostriction properties of the transducer core material, and thusfor a high efficiency of generation of acoustic vibrations.

The source of acoustic vibrations according to the invention is simplewith respect to the circuit design, employs elements widely used inmodern electrical engineering, and provides for a high power outputalong with a high efficiency, high dependability and high operatingstability.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be further understood by reference to thefollowing description of a specific embodiment thereof taken inconjunction with the accompanying drawings, wherein:

FIG. 1 shows the relationship between a magnetostrictive force P andmagnetization M of a transducer core;

FIG. 2 is a block digram of a source of acoustic vibrations;

FIG. 3 (a, b, c, d and e) presents waveforms of electrical andmechanical signals at various points of a circuit of the source ofacoustic vibrations (with time plotted on the X-axis).

BEST MODE FOR CARRYING OUT THE INVENTION

The exact nature of the method for generation of acoustic vibrationsconsists in the following.

The winding of the magnetostriction transducer is fed with an excitationpulse electrical signal in the form of a unidirectional half-cycle of acosinusoidal voltage. A pulse of current produced in the winding sets upa magnetic field which magnetizes the core material, with the resultthat a magnetostrictive force is produced which tends to alter thelength of the core. Variation of the core length with time depends onthe natural frequency of transducer mechanical oscillations and on theacoustic load impedance, and is vibratory in nature.

For increasing the amplitude of the transducer mechanical vibrations, itis expedient that the magnetostrictive force P is raised to a levelclose to saturated conditions P_(max) (FIG. 1).

Since the dependence of the magnetostrictive force P on themagnetization M is expressed by P=f(M.sup.α), where M is the transducercore magnetization, and α is in the range of 1 to 2, up to the saturatedcondition, for the majority of magnetostriction materials, the durationof cosinusoidal voltage half-cycles is taken to be equal to αhalf-cycles of acoustic vibrations produced by the loaded transducer.

In the case of linear dependence of the magnetostrictive force on themagnetization, the cosinusoidal voltage half-cycle duration is taken tobe equal to one half-cycle of acoustic vibrations generated by theloaded transducer.

In the case of quadratic dependence P=f(M²), the duration of thecosinusoidal voltage half-cycle is taken to be equal to two half-cyclesof the acoustic signal generated by the loaded transducer.

If the duration of the excitation cosinusoidal voltage pulses isadjusted as described hereinabove, the natural frequency of themagnetostriction transducer is coincident with the maximum level of themagnetostrictive force spectrum.

On completion of the electrical pulse, the external magnetic field and,hence, the magnetostrictive force P proper disappear, and the vibratorysystem of the transducer continues changing the dimensions thereof byvirtue of the energy stored therein. The core material demagnetizes to alevel of residual magnetization equivalent to a residualmagnetostrictive force P_(o) (FIG. 1). When subsequent unidirectionalpulses of the excitation cosinusoidal voltage are applied, themagnetostrictive force varies in the range from P_(o) to P_(max).. Themagnetostriction curve section from P_(o) is P_(o) ', characterized bylow magnetostriction is not utilized.

The frequency of the excitation electrical pulses (f_(v)) is taken to beequal to, or multiple (n) of the frequency of acoustic vibrations (f₁)produced by the loaded transducer, (nf_(v) =f₁) with the result that themagnetostrictive force acts in step with the transducer vibrations. Suchoperation is possible because each subsequent pulse of the cosinusoidalvoltage is applied to the transducer winding at the instant when thetransducer vibrating together with the load is in state whichcorresponds to a transfer from the negative region to the positive one(FIG. 3 e). The vibration amplitude of the core (that is, the amplitudeof the acoustic vibrations) gradually increases. The amplitude increasesfrom one pulse to another till the energy each time added to thetransducer becomes equal to the energy of radiation and losses occuringin the transducer during the same time period.

After the cosinusoidal voltage pulses supplied to the winding of themagnetostriction transducer are cut off, the vibrations thereofgradually converge.

The source of acoustic vibrations for realizing the method forgeneration of acoustic vibrations, according to the invention, comprisesa magnetostriction transducer 1 (FIG. 2), a main winding 2 and anauxiliary winding 3 disposed on the core 4 thereof and connected by theaiding connection method. The main winding 2 is connected through apower circuit of a main switching element 5 to plates of a reservoircapacitor 6. The source also incorporates a power unit 7 connectedthrough a power circuit of an auxiliary switching element 8 and throughthe auxiliary winding 3 to the plates of the capacitor 6. Connected tocontrol circuits of the switching elements 5 and 8 by the outputsthereof is a control unit 9 which controls the switching elements, andwhich is connected by one input thereof to an output of a pulserepetition frequency unit 10, the other input whereof is connectedthrough a phase-inverting circuit 11 to a feedback transmitter 12. Thelatter can be in the form of a piezoelectric or electromagnetictransducer mechanically coupled with the core 4. The switching elements5 and 8 may be in the form of gate thyristors. The pulse repetitionfrequency control unit 10 may be hooked around a self-excited oscillatoror an external-triggering one-shot multivibrator.

The switching element control unit 9 may be hooked around a symmetricone-shot multivibrator synchronized by a signal supplied from thefeedbaack transmitter 12.

During vibration of the magnetostriction transducer 1, the feedbacktransmitter 12 puts out an electrical signal corresponding to themechanical vibrations of the transducer 1.

If the frequency of control pulses set up by the switching elementcontrol unit 9 coincides with the natural frequency of the loadedtransducer 1, the synchronizing signal fed from the output of thephase-inverting circuit 11 does not affect the repetition frequency ofthe control pulses. If the repetition frequency of the control pulsesdeparts from the natural vibration frequency of the loaded transducer 1,the signal derived from the output of the phase-inverting circuit 11appropriately changes the control pulse repetition frequency and thuspermits the magnetostriction transducer 1 to operate on its resonantfrequency and to radiate a maximum of acoustic energy.

The source of acoustic vibrations operates as follows. The pulserepetition frequency control unit 10 produces a pulse for triggering theswitching element control unit 9. The unit 9 generates a control pulseU₁ (FIG. 3 a) which triggers the auxiliary switching element 8 (FIG. 2)serving to connect the reservoir capacitor 6 to the output of the powerunit 7 through the auxiliary winding 3. The reservoir capacitor 6charges with a current I (FIG. 3 c) flowing from the power unit 7 (FIG.2) through the auxiliary switching element 8 and winding 3 of thetransducer 1. The charge is oscillatory in nature, and the variation ofa voltage U₂ (FIG. 3 d) across the windings 2 and 3 of the transducer 1is nearly a cosinusoidal half-cycle. The resulting force P produced inthe megnetostriction material of the core 4 sets the vibratory system ofthe transducer 1 in motion. The most intense vibrations occur on thefrequencies close to the natural frequencies which depend on theequivalent mass, elasticity of the transducer 1, and the load impedance.For increasing the amplitude of acoustic vibrations, the duration of theelectrical excitation pulse (FIGS. 3 c, d) depending on the capacitanceof the reservoir capacitor 6 an on the inductance of one winding 2 or 3(FIG. 2), considering nonlinear character of the magnetostrictioncharacteristic and the effects of the transducer mechanical section onthe electrical section in transient process, is selected in the rangefrom one to two half-cycles of the loaded transducer resonant frequency.At the instant when a current I (FIG. 3 c) drops to zero, the switchingelement 8 (FIG. 2) is shorted out, and the transducer 1 starts vibratingfreely. During this time interval, the voltage U₂ (FIG. 3 d) is inducedacross the windings 2 and 3 due to an inverse magnetostriction effect.When the variation of vibrations produced by the transducer 1 passeszero level in the direction from the negative region to the positive one(FIG. 3 e), the control unit 9 (FIG. 2) produces a triggering pulse U₃(FIG. 3 b) for starting the switching element 5 (FIG. 2) whichdischarges the reservoir capacitor 6 through the main winding 2 of thetransducer 1. Both the charge and discharge of the capacitor 6 areoscillatory in nature, and continue during a time close to a chargingtime of the capacitor 6 (FIG. 3 c). Since both windings 2 (FIG. 2) and 3are inserted by aiding connection, the current I discharged by thecapacitor 6 induces a magnetic field in the core 4 directed identicallyto that induced during the charge of the capacitor 6. Thus, thetransducer 1 is energized in step with the vibrations thereof, with theresult that the vibrations are increased, and a unidirectionalcosinusoidal half-cycle voltage pulse is induced in the windings 2 and 3of the transducer 1.

At the end of the pulse of the current I (FIG. 3 c), the switchingelement 5 is disconnected, and the capacitor 6 acquires a charge ofopposite polarity in relation to the power source 7. During a repeatedtriggering of the switching element 8, the pulse of the current I (FIG.3 c), and the voltage U₂ in the transducer windings 2 and 3 (FIG. 3 d)increase as compared to the previous charging cycle of the capacitor 6,along with a further increase in the amplitude of vibrations (FIG. 3 e)of the transducer 1 (FIG. 2).

The vibration amplitude rises during alteration of charges anddischarges of the capacitor 6 through the windings 2 and 3 of thetransducer 1 until the energy added and the energy spent within the sametime become equal. After the pulse repetition frequency control unit 10stops operating, and the switching element control unit 9 stop producingthe pulses U₁ and U₂ (FIGS. 3 a, b), and at the instant of time when thecurrent I (FIG. 3 c) through the windings 2 (FIG. 2) and 3 and throughthe switching elements 5 or 8 drops to zero, both switching elements 5and 8 remain in non-conductive state, and the vibrations of thetransducer 1 gradually converge (FIG. 3 e). During generation of thenext pulse of all acoustic vibrations, the above processes are repeated.

It is possible that the source operates under conditions when theswitching elements operating during generation of acoustic pulses orcontinuous vibrations are actuated with a frequency which is lower thanthat of acoustic vibrations, but is multiple of it. Therefore,excitation pulses are delivered to the transducer at a frequency f_(v)whereby nf_(v) =f₁ wherein n is an integer multiple and f₁ is theacoustic frequency of the loaded transducer. The source of acousticvibrations of the present invention permits increasing the efficiency ofexitation of the magnetostriction transducers by 3 times as compared toshock-excitation pulse sources.

As regards the power output, the source of acoustic vibrations accordingto the invention is analogous to the prior-art sources of acousticvibrations wherein the magnetostriction transducers are excited underlinear conditions.

INDUSTRIAL APPLICABILITY

The source of acoustic vibrations according to the present inventiondesigned for realizing the novel method for generation of acousticvibrations can most advantageously be used in various ultrasonicproduction processes, including ultrasonic cutting, ultrasonic descalingof heat-exchange apparatus, ultrasonic welding, medical practice, etc.

We claim:
 1. A method of generating acoustic vibrations based on theshock excitation of a loaded magnetostriction transducer by a pulseelectrical signal comprising the steps of:determining the period P ofacoustic vibrations by the loaded transducer and frequency f₁ thereofgenerated by the loaded transducer; producing unidirectional half-cyclesof cosinusoidal voltage excitation pulses having a duration d from oneto two half-cycles of said period P of acoustic vibrations (1/2P<d≦P)and having a repetition frequency f_(v) which is a multiple n of theacoustic vibration frequency(nf_(v) =f₁); and applying said excitationpulses to said transducer and converting said excitation pulses into amagnetization pulse in said transducer such that said magnetizationpulse has a duration and frequency substantially equal to the durationand frequency of said excitation pulses.
 2. A source of acousticvibrations comprising:a power unit; a magnetostriction transducer havinga main field winding and an auxiliary field winding disposed thereon,said auxiliary field winding connected by the aiding connection methodto said main field winding such that current flowing in respectivepredetermined directions through said main field winding and saidauxiliary field winding induces a unidirectional magnetic field in saidtransducer; a first and a second switch means; a reservoir capacitorcoupled in parallel with the combination of said main field winding andsaid first switch means and coupled in parallel with the combination ofsaid auxiliary field winding, said second switch means and said powerunit; a pulse repetition frequency control means; and a control means,actuated by said pulse repetition frequency control means, forgenerating control signals and applying said control signals to saidfirst switch means and to said second switch means to actuate the sameand to establish a plurality of substantially consinusoidal half-cycleexcitation voltage pulses respectively across said auxiliary fieldwinding upon closure of said second switch means and across said mainfield winding upon closure of said first switch means wherein each saidexcitation voltage pulse has a duration from one to two half-cycles ofthe natural vibratory frequency of the loaded transducer and arepetition frequency which is a integer multiple of said naturalfrequency.